The effect of increased blood glow on tissue PO2 in various layers of the small intestine will be measured in non-absorptive and glucose absorption states. The purpose of the study is two fold: (1), to evaluate the existance of a counter-current exchange of oxygen in intestinal villi and (2), to determine the independant effects of changes in tissue partial pressure of oxygen. All studies will be done on exteriorized loops of small intestine from anesthetized rats. Diameters of microvessels will be measured with a video-microscope system and tissue PO2 will be measured with micro-oxygen electrodes. Blood flow will calculated by multiplying the cross-sectional area of a vessel times the average red cell velocity. If the tissue PO2 is higher at the base than apex of the villi, the existance of a counter-current mechanism is likely. In addition, if for equal blood flows, the tissue PO2 is lower during absorption of glucose than non-absorptive states, a lack of oxygen may participate in the vasodilation during glucose absorption.